Machines for handling sheet material, such as paper, commonly start out as a continuous web of material; an example is a printing press which although it may eventually produce printed sheets, may start out from roll or web stock fed into the press. Printing presses and similar machines may perform a multitude of operations on the continuous web of material, other than printing, such as slitting, punching, folding, and stacking sheets of the printed material. In addition, many paper products, such as advertising fliers, include perforations around coupons or other segments designed to be intentionally detached by the reader from the sheet, and to so such work many presses can be fitted with devices to perforate the sheet adjacent such coupon etc.
In the process of perforating sheet material, small holes and/or slits are actually punched through or formed in the material, leaving a small ridge or bump along the line of perforations. This ridge or bump is formed, to a large extent, by bits of the perforated material which remain attached to the sheet after the perforation process.
On a single sheet of perforated material, the ridge or bump formed along the perforation line is insignificant. However, in most operations, the continuous web of perforated material is subsequently cut into individual sheets and, then, numerous sheets are stacked vertically in a device at the end of the press called a sheeter-stacker. In a stack of hundreds or thousands of individual sheets the effect of the individual small ridges is multiplied to the extent that the stack becomes deformed away from the side the perforation is on and difficult or impossible to handle. Therefore, during operation of an automatic press, the number of sheets of perforated material in a stack must be limited and/or the stack must be adjusted periodically by hand to compensate for the stack-deforming perforation line. As a result, the production output of the press, measured in cutoffs (i.e., individual sheets) per hour, must be reduced significantly below the capabilities of the press operating in a fully automatic mode. In fact, a 40% to 60% reduction in press output is not unusual when running a perforation operation. Furthermore, deformed stacks of perforated sheets cause handling problems during subsequent operations, such as feeding individual sheets from the stack for cutting, folding, or in subsequent bindery operations.
All of the handling problems caused by the ridges or bumps formed on perforated sheets result in lower press production speed (cutoffs per hour), more frequent paper jams, decreased bindery production, more manual handling, and greatly increased production cost per unit of output. Therefore, there is a great need for a simple, effective, and efficient method and apparatus to permit perforated sheets to be placed in high stacks, transported, and manipulated by automatic equipment at high rates of speed, without the need for slow downs and/or special manual handling.